Magnetic and MO media are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval purposes. A magnetic medium in, e.g., disk form, such as utilized in computer-related applications, comprises a non-magnetic disk-shaped substrate, e.g., of glass, ceramic, glass-ceramic composite, polymer, metal, or metal alloy, typically an aluminum (Al)-based alloy such as aluminum-magnesium (Al—Mg), having at least one major surface on which a layer stack or laminate comprising a plurality of thin film layers constituting the medium are sequentially deposited. Such layers may include, in sequence from the substrate deposition surface, a plating layer, e.g., of amorphous nickel-phosphorus (Ni-P), a polycrystalline underlayer, typically of chromium (Cr) or a Cr-based alloy such as chromium-vanadium (Cr—V), a magnetic layer, e.g., of a cobalt (Co)-based alloy, and a protective overcoat layer, typically of a carbon (C)-based material, e.g., diamond-like carbon (“DLC”) having good tribological properties. A similar situation exists with MO media, wherein a layer stack or laminate is formed on a substrate deposition surface, which layer stack or laminate typically comprises a reflective layer, e.g., of a metal or metal alloy, one or more rare-earth thermo-magnetic (RE-TM) alloy layers, one or more transparent dielectric layers, and a protective overcoat layer, e.g., a DLC layer, for functioning as reflective, transparent, writing, writing assist, and read-out layers, etc.
Thin film magnetic and MO media in disk form, such as described supra, are typically lubricated with a thin topcoat film or layer comprised of a polymeric lubricant, e.g., a perfluoropolyether, to reduce wear of the disc when utilized with data/information recording and read-out transducer heads operating at low flying heights, as in a hard disk system functioning in a contact Start/Stop (“CSS”) mode. Conventionally, the thin film of lubricant is applied to the disc surface(s) during manufacture by dipping into a bath containing a small amount of lubricant, e.g., less than about 1% by weight of a fluorine-containing polymer, dissolved in a suitable solvent, typically a perfluorocarbon, fluorohydrocarbon, or hydrofluoroether.
Thin film magnetic recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (“CSS”) cycle commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk, and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., in order to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a “head crash.” Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
The lubricity properties of disk-shaped recording media are generally measured and characterized in terms of dynamic and/or static coefficients of friction. The former type, i.e., dynamic friction coefficient, is typically measured utilizing a standard drag test in which the drag produced by contact of a read/write transducer head with a disk surface is determined at a constant spin rate, e.g., 1 rpm. The latter type, i.e., static coefficients of friction (also known as “stiction” values), are typically measured utilizing a standard contact start/stop (“CSS”) test in which the peak level of friction is measured as the disk starts rotating from zero (0) rpm to a selected revolution rate, e.g., 7,200 rpm. After the peak friction has been measured, the disk is brought to rest, and the start/stop process is repeated for a selected number of start/stop cycles. An important property of a disk which is required for good long-term disk and drive performance is that the disk retain a relatively low coefficient of friction after many start/stop cycles or contacts with the read/write transducer head, e.g., 20,000 start/stop cycles.
According to conventional practices, a lubricant topcoat is uniformly applied over the protective overcoat layer to prevent wear between the disk and the facing surface of the read/write transducer head during CSS operation because excessive wear of the protective overcoat layer increases friction between the transducer head and the disk, eventually leading to catastrophic failure of the disk drive. However, an excess amount of lubricant at the head-disk interface causes high stiction between the head and the disk, which stiction, if excessive, prevents starting of disk rotation, hence catastrophic failure of the disk drive. Accordingly, the lubricant thickness must be optimized for stiction and friction.
The continuing requirements for increased recording density and faster data transfer rates necessitating lower flying heights of the data transducing heads and friction/stiction of the head-disk interface have served as an impetus for the development of specialized lubricants for serving as the lubricant topcoat layer overlying the protective overcoat layer. Such lubricants are required to fulfill a variety of functions requiring diverse characteristics and attributes. For example, the lubricant material forming the topcoat layer must be chemically inert, have a low vapor pressure, low surface tension, high thermal stability, mechanical stability under shear stress, and good boundary lubrication properties. In addition to the foregoing, it is critical that the lubricant adhere tightly to the underlying surface, i.e., the protective overcoat layer (typically carbon-based) over the lifetime of the disk drive comprising the recording disk and associated flying head data transducer.
Fluoropolyether lubricants have been developed which are in widespread use as materials for forming the lubricant topcoat layers of disk-shaped data/information storage and retrieval media, e.g., magnetic and MO recording media. Fluoropolyether-based lubricants are uniquely suited for use as lubricant topcoat layers in such rotating disk-type recording media in view of their exemplary properties, including inter alia, chemical inertness, low vapor pressure, low surface tension, high thermal stability, mechanical stability under high shear stress, and good boundary lubrication properties. Among the many currently available fluoropolyether lubricants, liquid perfluoropolyethers (PFPE) and derivatives thereof are most commonly employed in forming lubricant topcoat layers on rotatable recording media.
Commercially available PFPE lubricants include KRYTOX™ (DuPont Co., Wilmington, Del.); FOMBLIN™ Z-DOL, Z-TETRAOL, Z-DOL TXS, AM 2001, and AM 3001 (Ausimont Montedison Group, Thorofare, N.J.); and DEMNUM™ SA, SH, SP, and SY (Daikin Industries, Ltd., Osaka, Japan). A number of these commercially available PFPE-based lubricants are substituted with 2 to 4 polar end-groups, such as 2-4 hydroxyl or carboxyl groups, which polar end-groups are provided for enhancing adhesion of the polymeric lubricant molecules to the surface of the recording media, e.g., the surface of a carbon-based protective overcoat layer. Application of the PFPE-based lubricants to the media surface may be performed by any standard technique, e.g., dipping, spraying, spin coating, etc., followed by drying to remove any volatile solvent(s) therefrom, and if desired, followed by tape burnishing. A more recently developed technique for applying thin films of polymeric lubricants to media surfaces comprises generation of a vapor of the lubricant followed by condensation of the vapor on the media surface.
As indicated above, conventional PFPE-based lubricants generally comprise 2-4 polar groups or moieties at either end of a generally linear perfluorinated alkylpolyether molecule, for facilitating direct bonding to a surface, and thus, provide improved adhesion of the lubricant topcoat layer to the surface of the protective overcoat layer. Such polar functional groups, however, are not necessarily chemically inert, i.e., they may exhibit varying degrees of chemical inertness, and consequently, the above-described conventional PFPE-based lubricants may disadvantageously undergo chemical reactions prior or subsequent to their application to the media surface. In particular, contamination of the lubricant topcoat layer with a Lewis acid, e.g., aluminum oxide (Al2O3), may promote rapid degradation of the lubricant topcoat layer.
As a consequence of such concerns and problems associated with the chemical stability of derivatized PFPE lubricant materials utilized as topcoat layers in rotatable disk recording media, decomposition resistance of the lubricant is an important criterion in lubricant selection, particularly resistance against acid-catalyzed decomposition.
An approach for improving the performance of the most commonly employed lubricants utilized with thin film, disk-shaped magnetic and MO media, i.e., perfluoropolyether (“PFPE”)-based lubricants, which lubricants perform well under ambient conditions but not under conditions of higher temperature and high or low humidity, is described in, for example, U.S. Pat. No. 5,587,217, the entire disclosure of which is incorporated herein by reference, wherein it is disclosed that studies indicate that the tribological properties, and perhaps corrosion resistance, of perfluoropolyether-based lubricants utilized in the manufacture of thin film recording media can be substantially improved by addition thereto of an appropriate amount of a cyclotriphosphazene-based lubricant additive, e.g., a polyphenoxy cyclotriphosphazene comprising substituted or unsubstituted phenoxy groups, to form what is termed a “composite lubricant”.
Currently, bis (4-fluorophenoxy)-tetrakis (3-trifluoromethyl phenoxy) cyclotriphosphazene (available as X-1P™ from Dow Chemical Co., Midland, Mich.) is the additive most commonly utilized with perfluoropolyether-based lubricants for forming composite lubricants for use with thin film magnetic and MO media. However, as disclosed in U.S. Pat. Nos. 5,718,942 and 5,908,817, the disclosures of which are incorporated herein by reference, the use of X-1P as a lubricant additive for forming composite lubricants comprising the perfluoropolyether-based lubricants commonly employed in the data storage industry (e.g., Fomblin Z-DOL™ and Fomblin Z-TETRAOL™, each available from Ausimont, Thorofare, N.J.) incurs a disadvantage in that the former (i.e., the cyclotriphosphazene-based lubricant additive) has very low solubility in the latter (i.e., the PFPE-based primary lubricant), resulting in phase separation and droplet formation.
Another approach for improving the performance of the most commonly employed lubricants utilized with thin film, disk-shaped magnetic and MO media, i.e., perfluoropolyether (“PFPE”)-based lubricants is the addition of phosphate and/or phosphite antioxidant/stabilizer materials to solutions of the PFPE-based lubricants. However, while such additions have been found to be very effective in retarding chemical degradation of the PFPE-based lubricant thin films, the addition of such antioxidant/stabilizer materials incurs a number of problems, including phase separation, hazing, and poor tribological performance, especially with ultra-thin lubricant films.
Thus, a significant factor in evaluating the performance of candidate lubricant materials for use as lubricant topcoat layers in rotatable recording media is the ability of the lubricant to resist chemical decomposition over time, particularly acid-catalyzed decomposition, while not deleteriously affecting critical or requisite lubricant properties. In view of the criticality of the lubricant topcoat in obtaining and maintaining optimum performance of rotating disk recording media utilized with flying head read/write transducers operating at very low flying heights, there is a continuing need for lubricant materials and topcoat layers exhibiting improved chemical and physical durability, corrosion resistance, adhesion, stiction and wear performance, etc., particularly under conditions of high stress, temperature, and humidity.